The broad goal of the proposed research is to learn more of the molecular nature of genes and enzymes. This will be pursued by two lines of study which overlap since the same experimental system, the tryptophan operon of Salmonella typhimurium, will be exploited. First, studies into the nature of the interaction of the structural genes and the regulation and initiating elements of the operon will be undertaken. Specifically, the structure and function of the promoters, both natural and mutagen-induced, will be examined using fine structure genetic and biochemical approaches. These studies will be greatly facilitated by the availability of a set of operator-promoter deletions which dissect the trp operon from its operator end. Using these deletions and applying biochemical and genetic methods an experimental analysis of the molecular basis of various mechanisms by which reinitiation of gene expression can be accomplished is planned with the intent of uncovering the molecular structure of genetic promoter elements. The second facet of the proposal is concerned with the interplay of subunit structure and function in the intergrity of large multifunctional enzyme complexes. The experimental system for this study will be the anthranilate synthetase-phosphoribosyl transferase complex of tryptophan biosynthesis in Salmonella typhimurium. This complex catalyzes the first two reactions of the pathway and is regulated allosterically by end product inhibiton. A study of the two component subunits of the complex of wild type and of mutant strains, is proposed with the objective being to learn further the role each plays in the process of assembly and in the establishment of the integrity of the complex. Structural, functional and mutational analyses will be employed to test possible modes of enzyme evolution which are operative in the generation of more sophisticated enzymes from simpler ones.